Context : When can an active galactic nucleus ( AGN ) be considered radio-loud ?
Following the established view of the AGNs inner workings , an AGN is radio-loud if associated with relativistic ejections emitting a radio synchrotron spectrum ( i.e. , it is a “ jetted ” AGN ) .
In this paper we exploit the AGN main sequence that offers a powerful tool to contextualize radio properties .

Aims : If large samples of optically-selected quasars are considered , AGNs are identified as radio-loud if their Kellermann ’ s radio loudness ratio R _ { \mathrm { K } } > 10 .
Our aims are to characterize the optical properties of different classes based on radio-loudness within the main sequence and to test whether the condition R _ { \mathrm { K } } > 10 is sufficient for the identification of RL AGNs , since the origin of relatively strong radio emission may not be necessarily due to relativistic ejection .

Methods : A sample of 355 quasars was selected by cross-correlating the FIRST survey with the SDSS DR14 quasar catalog .
We classified the optical spectra according to their spectral types along the main sequence of quasars .
For each spectral type , we distinguished compact and extended morphology ( providing a FIRST-based atlas of radio maps in the latter case ) , and three classes of radio-loudness : detected ( specific flux ratio in the g band and at 1.4GHz , R _ { \mathrm { K ^ { \prime } } } < 10 ) , intermediate ( 10 \leq R _ { \mathrm { K ^ { \prime } } } < 70 ) , and radio loud ( R _ { \mathrm { K ^ { \prime } } } \geq 70 ) .

Results : The analysis revealed systematic differences between radio-detected ( i.e. , radio-quiet ) , radio-intermediate , and radio-loud in each spectral type along the main sequence .
We show that spectral bins that contain the extreme Population A sources have radio power compatible with emission by mechanisms ultimately due to star formation processes .
Radio-loud sources of Population B are characteristically jetted .
Their broad H \beta profiles can be interpreted as due to a binary broad-line region .
We suggest that RL Population B sources should be preferential targets for the search of black hole binaries , and present a sample of binary black hole AGN candidates .

Conclusions : The validity of the Kellermann ’ s criterion may be dependent on the source location along the quasar main sequence .
The consideration of the MS trends allowed to distinguish between sources whose radio emission mechanisms is “ jetted ” from the ones where the mechanism is likely to be fundamentally different .